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Injury response is key to successful regeneration. Yet, transcriptome analyses of injury response were performed only on a handful of regenerative organisms. Here, we studied the injury response of the solitary ascidian Polycarpa mytiligera, an emerging model system, capable of regenerating any body part. We used the siphon as a model for studying transcriptional changes following injury, and identified genes that were activated in the initial 24 hours post amputation (hpa).Highly conserved genes, such as bone morphogenetic protein-1 (BMP1), growth hormone secretagogue receptor (GHSR) and IL-17, were upregulated by 12 hpa, yet their expression was sustained only in non-regenerating tissue fragments. We optimized fluorescent in situ hybridization, and found that the majority of BMP1+ cells were localized to the rigid tunic that covers the animal. This highlights the importance of this tissue, particularly during injury response. BMP1 was overexpressed following injuries to other body regions, suggesting that it was a part of a common injury-induced program.Our study suggests that, initially, specific injury-induced genes were upregulated in P. mytiligera organs, yet, later, a unique transcriptional profile was observed only in regenerating tissues. These findings highlight the importance of studying diverse regenerating and non-regenerating organisms for complete understanding of regeneration.

The word ‘plastic’ originates from the Ancient Greek ‘plastikos’, meaning shapeable, moldable. This trait has significantly contributed to its popularity and rapid global increase in use in recent decades, while also presenting immense challenges for its detection, identification, and quantification in marine ecosystems. Here, we review the current methods in marine plastic assessment and present the main challenges in monitoring plastic pollution in marine environments. We also focus on plastic and its additives’ interactions with biological systems and the urgent need to establish standardization of the methodologies and techniques worldwide to enable accurate evaluations and comparisons of plastic pollution and further our understanding of its impact on marine organisms and ecosystems.

Suspension feeders, including ascidians (Phylum Chordata, Class Ascidiacea), experience a dilute prey field composed of extremely small particles. The filtration apparatus of ascidians is based on a mucous-mesh that is continuously secreted and ingested. The rate and metabolic cost of this mesh secretion has not been quantified to date. We used video boroscopy to quantify the mucous-mesh production rate of the solitary ascidian Herdmania momus under different food and temperature treatments. H. momus individuals with an average (±95% CI) biomass of 30.7 ± 1.1 mg and a branchial sac area of 10.3 ± 1.2 cm2 produced an average of 276 ± 33.5 cm2 of mucous-mesh h-1, corresponding to a median turnover rate of 625 ± 82 mesh d-1. Since the mean mesh mass was 2.44 ± 0.58 mg, this production rate corresponds to roughly 50 ± 8 times the individual’s biomass per day. Food concentration had no detectable effect on mesh production rate, whereas a temperature difference of ~9°C (20 vs. 29°C) moderately increased mesh production by 30-50%. The current study reveals that the feeding process of H. momus involves a high expenditure on mucous-mesh synthesis that, combined with low food availability, may limit its growth in oligotrophic waters and under changing climate regimes.

Rapid plastic response to environmental changes, which involves extremely complex underlying mechanisms, is crucial for organismal survival during many ecological and evolutionary processes such as those in global change and biological invasions. Gene expression is among the most studied molecular plasticity, while co- or post-transcriptional mechanisms are still largely unexplored. Using a model invasive ascidian Ciona savignyi, we studied multidimensional short-term plasticity in response to hyper- and hypo-salinity stresses, covering the physiological adjustment, gene expression, alternative splicing (AS), and alternative polyadenylation (APA) regulations. Our results demonstrated that rapid plastic response varied with environmental context, timescales, and molecular regulatory levels. Gene expression, AS, and APA regulations independently acted on different gene sets and corresponding biological functions, highlighting their non-redundant roles in rapid environmental adaptation. Stress-induced gene expression changes illustrated the use of a strategy of accumulating free amino acid under high salinity and losing/reducing them during low salinity to maintain the osmotic homoeostasis. Genes with more exons were inclined to use AS regulations, and isoform switches in functional genes such as SLC2a5 and Cyb5r3 resulted in enhanced transporting activities by upregulating the isoforms with more transmembrane regions. The extensive 3’-untranslated region (3’UTR) shortening through APA was induced by both salinity stresses, and APA regulation predominated transcriptomic changes at some stages of stress response. The findings here provide evidence for complex plastic mechanisms to environmental changes, and thereby highlight the importance of systemically integrating different levels of regulation mechanisms in studying initial plasticity in evolutionary trajectories.

SummaryCentral nervous system (CNS) regeneration extent is highly diverse across the metazoans, with adult mammals demonstrating limited ability1,2. Understanding how neurons regenerate following injury remains a central challenge in regenerative medicine. Although conserved pathways associated with neural regeneration have been identified3,4, a study describing the stepwise morphogenetic changes that take place throughout a complete CNS regeneration is lacking. Utilizing the highly regenerative tunicate model Polycarpa mytiligera5, we characterized the morphological, cell proliferation, and transcriptomic dynamics that lead to entire CNS regeneration. The regenerated CNS of adult P. mytiligera expressed key neurodevelopmental markers that are not otherwise present in the adult CNS. Removal of the entire CNS resulted in high cell proliferation in the regenerated area. Transcriptome analysis revealed enhanced stem-cell related gene activity, with high expression of P53 and piRNA pathways preceding the activation of Notch, Wnt, and Nanos pathways. The CNS regeneration atlas created here depicts the transcriptomic landscape of the entire CNS regeneration process, revealing the core pathways that regulate neuronal response to injury, and the regeneration stage at which they are most pronounced. The molecular and cellular mechanisms controlling regenerative capacity that this atlas reveals could be used to develop approaches to enhancing neurogenesis in closely-related chordate species, including humans.

Plastic additives (PAs) are chemical compounds incorporated into the plastic during the manufacturing process. Phthalate acid esters, bisphenols, and nonylphenols are all PAs found in marine environments and associated with endocrine-disrupting processes. However, our knowledge regarding the impact of endocrine-disrupting PAs on coral-reef organisms is limited. As reef population structure is directly linked to reproduction and larval settlement processes, interference with hormonal systems can impact coral-reef community structure, particularly if the effects of PAs differ among species. In the current study we exposed the reproductive products of four tropical coral-reef invertebrates to environmentally-relevant concentrations of four prevalent PAs in seawater: dibutyl phthalate (DBP), dimethyl phthalate, (DMP), 4-nonylphenol (4-NP), and bisphenol A (BPA), as well as to 10

This study represents, detailly, the validated method for the extraction and quantification of widespread phthalic acid esters (PAEs) bis(2-ethylhexyl) phthalate (DEHP), di-n-butyl phthalate (DBP) and di-n-octyl phthalate (DnOP) from solitary ascidians collected from a marine environment. The extraction was based on a pressurized liquid extraction method, using n-hexane as the solvent to extract the target PAEs from dry biological tissues, and was performed in an accelerated solvent extraction instrument. The average recovery of 89.2% was obtained from samples subjected to a pressure of ~1500 psi and 120°C in two 10-min cycles. GC-MS was used for quantification, conducted in single-ion monitoring mode. Following careful and rigorous cleanup procedures to prevent cross-contamination from laboratory glassware, PAE standards showed signals with good specificity. The obtained limits of detection were 130, 122 and 89 ng/g for DEHP, DBP and DnOP, respectively. Accordingly, the calculated limits of quantification were 394, 370 and 270 ng/g for DEHP, DBP and DnOP, respectively. The obtained linearity ranged from 5.4 to 269 ng/ml (equivalent to 135 - 6725 ng/g dry weight), with R2 ≥ 0.998. Concentrations in the range of 200 to 9000 and 400 to 5000 ng/g sample dry weight, for DEHP and DBP, respectively, were obtained from the ascidians. No DnOP was detected in any of the samples. These results indicate that the method presented in this study is applicable for detection of low and trace concentrations of the target PAEs in samples collected from a marine organism, which can serve as a bioindicator of plastic contamination.

The shipping industry is considered the main vector of introduction of marine non-indigenous species (NIS). NIS distributions are often a consequence of frequent trade activities that are affected by economic trends. A dominant trend in the shipping industry is the operation of Ultra Large Container Vessels (ULCV), which are over 395 m long and sail mostly on the East-Asia - northern-Europe route. Understanding the risk of NIS introduction by this emerging shipping category is needed for devising strategies for sustainable shipping. Here, we conducted a controlled simulation of key abiotic factors that determine marine bioinvasion success: temperature, salinity, and food availability along selected routes, under two treatments: ULCV and intermediate-size vessels. We tested the effect of each treatment and the varying environmental conditions on the survival of two invasive ascidians (Chordata, Ascidiacea). We used survival analysis methods to locate predictors of ascidian mortality; Environmental conditions at ports with high mortality were used to identify similar major ports on a global scale as potential abiotic barriers. The key factors in ascidian mortality varied between the two species, but for both species, the treatment and salinity were dominant predictors for survival. We identified Port Klang, Rotterdam, and Dammam as ports with high mortality and located several globally distributed major ports that present similar environmental conditions. Our results highlight the potential role of selected major ports as abiotic barriers to fouling organisms during ocean voyages. The tolerance of the tropical-origin Microcosmus exasperatus to the northern-Europe conditions, and of the temperate/sub-tropical origin Styela plicata, to high temperature conditions, point out the urgent need to modify international fouling regulations in view of global change. Further studies on the survival of fouling organisms during a cascade of changing environmental conditions will contribute to the advancement of science-based regulations to reduce the adverse effects of NIS.

Introductions of non-indigenous ascidian species are highly common nowadays due to their ability to establish successfully on artificial substrates. Didemnid species are of particular concern because of their rapid a-sexual growth and high fecundity. The colonial ascidian Didemnum perlucidum Monniot F., 1983 was first described from Guadeloupe Island in the Caribbean, although its native range remains unknown. To date, it has been recorded from numerous sites across the Indo-Pacific and Atlantic oceans. Here, having employed both classic taxonomy and genetic tools to verify its identification, we document for the first time its occurrence in the Mediterranean Sea – on two artificial substrates along the Israeli coast. The ability of D. pelucidum to establish reproductive populations despite the harsh environmental conditions of this region, with temperature fluctuations between 16-31°C and a salinity of 38-39 ppt, raises concern regarding this species’ potential for introductions at numerous sites across the Mediterranean.

The disproportionately low presence of marine species in the list of invasive alien species (IAS) of Union concern of the European Union (EU) Regulation 1143/2014 does not fully acknowledge the threat they pose to the EU marine environment. In this study, the first EU-scale Horizon Scanning (HS) focusing on marine alien species was performed, aiming to deliver a ranked list of species that should be of high priority for risk assessment (Article 5 of the EU IAS Regulation). Species absent from or with a limited distribution in EU marine waters were targeted. In total, 363 alien species were initially screened for HS by a panel of experts, including a broad range of taxonomic groups. Species were scored for their likelihood of arrival, establishment, spread, and impact in EU waters. A consensus workshop ranked 267 species, including a subset of 26 prioritized species. These species are considered to be mainly introduced by shipping (fouling and ballast water), via the Suez Canal, and aquaculture activities. The 26 priority species were also scrutinized in terms of feasibility of their management; 18 of them were suggested for performing risk assessments on the basis of the EU IAS Regulation. Since biological invasions are dynamic and connected with accelerated globalization and diversified human activities, we recommend HS to be repeated periodically to review the species already listed and assess new ones.

The tropical ascidian Herdmania momus (Savigny 1816) has been rapidly expanding its distribution in the Eastern Mediterranean Basin. To examine the role of temperature resilience on its invasion success, we conducted monthly field surveys for two years at Achziv (Israel) Marine Protected Area, and examined the survival of adult individuals from native (Red Sea) and invasive (Mediterranean) populations under different temperature treatments. In addition, temperature effects on fertilization and larval development were examined in controlled laboratory conditions. Results indicate that temperature has a significant effect on H. momus from its early life stages through adulthood. Field surveys revealed a significant decline in H. momus abundance in the Mediterranean with the decrease in seawater temperature. Low temperature inhibited development of early life stages, and individuals from both populations demonstrated low survivability under low temperature treatment. All the above may be derived from the tropical origin of H. momus, and may further limit its dispersal into colder areas. However, adult individuals from the invasive population demonstrated significantly higher survivability to the high temperature treatment in comparison to the native population from the Red Sea. Larvae development, metamorphosis and settlement were enhanced and with higher rates of success under the high temperature conditions. The high-temperature adaptation of the Mediterranean population of H. momus from fertilization to adulthood may thus contribute to its expanding distribution in the Eastern Mediterranean.

The increasing use of pharmaceuticals in human and veterinary medicine, along with their poor removal rates at wastewater treatment facilities is resulting in the chronic release of pharmaceutically-active compounds (PhACs) into the marine environment, where they pose a threat to non-target organisms. A useful approach, as applied in the current study for assessing the effects of PhACs on non-target organisms, is the proteomic approach: the large-scale study of an organism's proteins. Using ‘shotgun’ proteomics, we identified differentially-expressed proteins based on peptide fragments in the solitary ascidian, Herdmania momus, following a 14-day laboratory experimental exposure to the PhAC carbamazepine (CBZ), an anticonvulsant and antidepressant medication, frequently detected in the aquatic environment. Individuals were exposed to environmentally relevant concentrations: 5 or 10 µg/L of CBZ, in addition to a control treatment. Out of 199 identified proteins, 24 were differentially expressed (12%) between the treatment groups, and thus can potentially be developed as biomarkers for CBZ contamination. Ascidians’ phylogenetic position within the closest sister group to vertebrates presents an advantage in examining the pathological effects of PhACs on vertebrate-related organs and systems. Together with the world-wide distribution of some model ascidian species, and their ability to flourish in pristine and polluted sites, they provide a promising tool through which to study the extent and effects of PhAC contamination on marine organisms.

In this study we present a list of invasive/potential invasive alien species in the East and South European Network for Invasive Alien Species (ESENIAS) countries with marine borders. The species were classified according to the existing literature and experts’ judgment, as established, casual, invasive and expected. Finally, factsheets were compiled for ten species of high importance based on their expanding/invading character. Of the 160 species comprising the list, 149 were already present in the ESENIAS countries, while eleven were invasive species either present in the Mediterranean or in other European Seas, likely to be recorded in the ESENIAS countries. The majority of the species were of Red Sea/IndoPacific origin (97 species; 60.6%). Italy, Turkey and Greece were the countries with the highest representation of species (159, 152 and 139 species respectively), due to their extended coastline and the number of scholars working on marine invasive species. The highest number of established species was recorded in Turkey (116 species), whereas in Italy and Greece the most numerous species were the “expected” ones (85 and 48 species, respectively). The eastern Adriatic Sea countries (i.e. Albania, Croatia, Montenegro and Slovenia) had generally low numbers of species in this list, many of which are still “expected” to arrive from the neighbouring countries of Greece and Italy. Finally, the most frequently potential pathway was transfer stowaways (ship ballast water: 41 cases; ship hull fouling: 55), whereas unaided spread of Lessepsian immigrants followed (95 cases). This list is intended to serve as an early warning system that through horizon scanning process would assist ESENIAS countries to prioritise invasive alien species, their pathways and the areas of higher likelihood to appear, in order to take management measures., U ovom radu predstavljamo popis invazivnih i potencijalno invazivnih vrsta na istoku i jugu Europske mreže za invazivne vrste (ESENIAS) u zemljama s morskim granicama. Vrste su klasificirane prema postojećoj literaturi i procjeni stručnjaka, pa su tako utvrđene grupe povremenih, invazivnih i očekivanih vrsta. Podaci su dati za deset vrsta čija se važnost temelji na njihovom širenju i invazivnom karakteru. Od 160 vrsta koje sadrži popis, 148 je već bilo prisutno u ESENIAS zemljama, dok je 9 invazivnih vrsta bilo prisutno u Sredozemnom ili u drugim europskim morima, a vjerojatno je da će se zabilježiti i u zemljama udruženim u ESENIAS. Većina vrsta je bile iz Crvenog mora / indopacifičkog podrijetla (97 vrsta, 60,6%). Italija, Turska i Grčka su zemlje s najvišom zastupljenošću vrsta (159, 152 i 139), zbog njihove proširene obale i broja znanstvenika koji rade na morskim invazivnim vrstama. Najveći broj utvrđenih vrsta zabilježen je u Turskoj (116 vrsta), dok su u Italiji i Grčkoj najbrojnije vrste bile “očekivane” (85 i 48 vrsta, respektivno). Istočne zemlje Jadranskog mora (npr. Albanija, Hrvatska, Crna Gora i Slovenija) imale su općenito nizak broj vrsta na ovom popisu, od kojih mnoge još “očekujemo” da pristignu iz susjednih zemalja: Grčke i Italije. Konačno, najčešće su potencijalni putovi bili „transferni putnici“ (balastna voda broda: 41 slučaj, obraštaj brodskog trupa: 55), dok je slijedilo i širenje lesepsijskih migranata (95 slučajeva). Ovaj popis je namijenjen da služi kao sustav ranog upozorenja koji bi kroz proces skeniranja pomogao državama ESENIAS da daju prioritet invazivnim stranim vrstama, njihovim putovima i područjima veće vjerojatnosti pojavljivanja, kako bi se poduzele potrebne mjere upravljanja.

The solitary ascidian Ciona intestinalis is among the most damaging of invasive fouling species in the world. It is mostly known from cold-water or temperate environments, where it is able to form dense aggregations. We present the first record of C. intestinalis type-A, now recognized as C. robusta, from the Eilat marina, Israeli Red Sea coast. Several individuals found fouling the bottom of the floating docks in March 2015 were identified using both mitochondrial sequencing and morphological tools. Periodical surveys revealed some of them with full gonoducts, and small individuals appeared two months later, indicating a reproductive population. The extremely high salinity and temperature at the Eilat marina indicates the wide environmental tolerance of this species. Since the only other report from this region dates to 100 years ago, from a dredging vessel at the entrance to the Suez Canal, and it has not since been recorded from any natural substrates in the region, it should be considered a non-indigenous species in the Red Sea.

Significance The mechanisms by which organisms form mineralized skeletons have been a major research focus for the last 50 y and remain so today. Among the most surprising discoveries is the recent observation that different animals use the same mechanisms, and precisely the same amorphous precursors, to form biomineralized structures as diverse as coral skeletons, molluscan shells, and sea urchin spines. In living animals, skeletal biomineralization from amorphous precursors correlates with a distinctive nanoparticulate texture that can be preserved in fossils, enabling us to probe mechanisms of skeletal formation in early animals. We document nanoparticulate texture in some of the oldest known carbonate skeletons, which strongly suggests that skeletons formed from amorphous precursors throughout the recorded history of animals., Crystallization by particle attachment (CPA) of amorphous precursors has been demonstrated in modern biomineralized skeletons across a broad phylogenetic range of animals. Precisely the same precursors, hydrated (ACC-H2O) and anhydrous calcium carbonate (ACC), have been observed spectromicroscopically in echinoderms, mollusks, and cnidarians, phyla drawn from the 3 major clades of eumetazoans. Scanning electron microscopy (SEM) here also shows evidence of CPA in tunicate chordates. This is surprising, as species in these clades have no common ancestor that formed a mineralized skeleton and appear to have evolved carbonate biomineralization independently millions of years after their late Neoproterozoic divergence. Here we correlate the occurrence of CPA from ACC precursor particles with nanoparticulate fabric and then use the latter to investigate the antiquity of the former. SEM images of early biominerals from Ediacaran and Cambrian shelly fossils show that these early calcifiers used attachment of ACC particles to form their biominerals. The convergent evolution of biomineral CPA may have been dictated by the same thermodynamics and kinetics as we observe today.

The solitary ascidian Microcosmus exasperatus (Class: Ascidiacea, Family: Pyuridae) is found in both tropical and sub-tropical waters. In the Mediterranean it has been recorded on the coasts of Israel, Lebanon, Turkey, and Tunisia. Here we report its first record from Cyprus, where it was found forming aggregations on ropes in a marina. It was also recorded there on the hull of a 7-meter pleasure craft. We provide both molecular and morphological diagnostics to facilitate the accurate identification of M. exasperatus, as its external appearance resembles that of M. squamiger, which is a widely-distributed species in the western Mediterranean. The occurrence of M. exasperatus on ship hulls and its ability to form large aggregations on artificial substrata may accelerate introduction of the non-native epifaunal organisms that cover its tunic, consequently altering local biodiversity. Thus, we strongly recommend monitoring the progression of this invasive species in the eastern Mediterranean.

Environmental factors, such as temperature and salinity, are known to influence distribution patterns and invasion success in ascidians. The solitary ascidian Microcosmus exasperatus Heller, 1878 has a wide global distribution and can be found in both tropical and sub-tropical waters. In the Mediterranean Sea, it is considered to be an invasive species introduced through the Suez Canal, with a restricted distribution in the eastern Mediterranean. Despite its global distribution, the environmental tolerances of this species are poorly known. We examined the effect of varying temperature and salinity on the survival of adult individuals of M. exasperatus in a laboratory setting to partially determine its environmental tolerance range. In addition, it’s global and local distribution as well as the seasonal abundance in ‘Akko Bay (northern Mediterranean coast of Israel) were examined. Field observations and laboratory experiments show that M. exasperatus is able to tolerate a temperature range of 12– 30 oC, and salinity of 37– 45, but it survived poorly in salinity of 33– 35 and temperatures > 32 oC. Considering this relatively wide tolerance range of M. exasperatus to temperature and salinity together with the anticipated rise in anthropogenic disturbances, we expect this species to further spread into new locations along the Mediterranean coast.

Global increase in the use of pharmaceutically-active compounds (PhACs), and their insufficient removal in wastewater treatment plants, have resulted in their continuous release into the marine environment. We investigated the use of the solitary ascidians Herdmania momus, Microcosmus exasperatus, and Styela plicata as bioindicators of three common PhACs in the Israeli coastal waters: Bezafibrate, carbamazepine and diclofenac. Both the Mediterranean and the Red-Sea coasts were found contaminated with PhACs, detected at all 11 sampling sites, with four sites contaminated with all three studied PhACs. Diclofenac was most frequent, present in nine of the 11 sites with concentrations reaching 51.9 ng/g of dry weight sample (dw). Bezafibrate and carbamazepine reached concentrations of 47.8 ng/g dw and 14.3 ng/g dw, respectively. The alarming detection of such high concentrations of PhACs in ascidians along Israel's coasts demonstrates both the extent of PhACs contamination in the region, and the potential of ascidians as bioindicators, and emphasizes the urgent need for additional research into PhAC contamination sources and effects.

Background Adaptive evolution is one of the crucial mechanisms for organisms to survive and thrive in new environments. Recent studies suggest that adaptive evolution could rapidly occur in species to respond to novel environments or environmental challenges during range expansion. However, for environmental adaptation, many studies successfully detected phenotypic features associated with local environments, but did not provide ample genetic evidence on microevolutionary dynamics. It is therefore crucial to thoroughly investigate the genetic basis of rapid microevolution in response to environmental changes, in particular on what genes and associated variation are responsible for environmental challenges. Here, we genotyped genome-wide gene-associated microsatellites to detect genetic signatures of rapid microevolution of a marine tunicate invader, Ciona robusta, during recent range expansion to the harsh environment in the Red Sea. Results The Red Sea population was significantly differentiated from the other global populations. The genome-wide scan, as well as multiple analytical methods, successfully identified a set of adaptive genes. Interestingly, the allele frequency largely varied at several adaptive loci in the Red Sea population, and we found significant correlations between allele frequency and local environmental factors at these adaptive loci. Furthermore, a set of genes were annotated to get involved in local temperature and salinity adaptation, and the identified adaptive genes may largely contribute to the invasion success to harsh environments. Conclusions All the evidence obtained in this study clearly showed that environment-driven selection had left detectable signatures in the genome of Ciona robusta within a few generations. Such a rapid microevolutionary process is largely responsible for the harsh environmental adaptation and therefore contributes to invasion success in different aquatic ecosystems with largely varied environmental factors. Electronic supplementary material The online version of this article (10.1186/s12862-018-1311-1) contains supplementary material, which is available to authorized users.

The interaction of microplastic (MP) with marine organisms is crucial for understanding the significant effect that MP and its additives may have on marine environments. However, knowledge regarding the magnitude of these pollutants in the Eastern Mediterranean Sea and the tropical Red Sea is still scarce. Here we examined the levels of phthalate acid esters (PAEs) and MP in Herdmania momus and Microcosmus exasperatus sampled along the Mediterranean and Red Sea coasts of Israel. High levels of dibutyl phthalate (DBP) and bis (2‑ethylhexyl) phthalate (DEHP) were found in ascidians at the majority of sampling sites, and MP particles were found in ascidians at all sites. As efficient filter-feeders and being widely-distributed, invasive ascidians present fundamental opportunities for the environmental monitoring of microplastic and its additives worldwide. The high levels of pollutants revealed emphasize the need for further research into the magnitude and effects of MP and PAEs in these regions.

Regeneration, the process of replacing lost or damaged body parts, has long captured human imagination and is a key feature among all animal phyla. Due to their close phylogenetic relationship to vertebrates and their high regenerative abilities, ascidians (Chordata, Ascidiacea) are often used as models to shed light on the cellular and genetic process involved in tissue regeneration. Surprisingly, ascidian regeneration studies are based on only a few model species. In this chapter, we point out the important potential of solitary ascidians in regenerative and stem cell studies. We review recent studies of regeneration among solitary ascidians and discuss the cellular mechanism of tissue regeneration and the possible involvement of circulating cells in these processes. New data regarding the relationship between age and regeneration abilities of the solitary ascidian Polycarpa mytiligera (Stolidobranchia, Styelidae) are presented. The unique regeneration abilities found in P. mytiligera following evisceration of its digestive system and following amputation of its neural complex and siphon-associated structures and nerves imply on its potential to serve as a novel model system for understanding tissue regeneration.

Ascidians (Chordata, Ascidiacea) are considered to be prominent marine invaders, able to tolerate highly polluted environments and fluctuations in salinity and temperature. Here, we examined the seasonal and spatial dynamics of the microbial communities in the inner-tunic of two invasive ascidians, Styela plicata (Lesueur 1823) and Herdmania momus (Savigny 1816), in order to investigate the changes that occur in the microbiome of non-indigenous ascidians in different environments. Microbial communities were characterized using next-generation sequencing of partial (V4) 16S rRNA gene sequences. A clear differentiation between the ascidian-associated microbiome and bacterioplankton was observed, and two distinct sets of operational taxonomic units (OTUs), one core and the other dynamic, were recovered from both species. The relative abundance of the dynamic OTUs in H. momus was higher than in S. plicata, for which core OTU structure was maintained independently of location. Ten and seventeen core OTUs were identified in S. plicata and H. momus, respectively, including taxa with reported capabilities of carbon fixing, ammonia oxidization, denitrification, and heavy-metal processing. The ascidian-sourced dynamic OTUs clustered in response to site and season but significantly differed from the bacterioplankton community structure. These findings suggest that the associations between invasive ascidians and their symbionts may enhance host functionality while maintaining host adaptability to changing environmental conditions.

Harbor systems represent passive gateways for the introduction of nonnative ascidians that compete with the surrounding benthos and may spread through localized dispersal, even populating adjacent natural reefs. To investigate the potential role of microbial symbionts in the success of ascidian introductions and spread, we evaluated the host-specificity of prokaryotic communities within two ascidian species commonly found off the North Carolina coast. Replicate samples of the native ascidian Eudistoma capsulatum, the nonnative ascidian Distaplia bermudensis and seawater were collected from artificial (harbor) and natural reef substrates. Prokaryotic communities in seawater samples and ascidian tunics were characterized via next-generation sequencing of partial 16S rRNA gene sequences. Ascidian microbiomes clustered strongly in response to host species, with significant differences in community structure between the two species and seawater. Further, symbiont community structure differed significantly between E. capsulatumindividuals collected from artificial and natural habitats, though this was not the case for D. bermudensis. These findings suggested that some ascidian species possess stable microbial symbiont communities that allow them to thrive in a wide range of habitats, while other species rely on the restructuring of their microbial communities with specific symbionts (e.g. Chelativorans) to survive under particular environmental conditions such as increased pollution.

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Regeneration, the process of replacing lost or damaged body parts, has long captured human imagination and is a key feature among all animal phyla. Due to their close phylogenetic relationship to vertebrates and their high regenerative abilities, ascidians (Chordata, Ascidiacea) are often used as models to shed light on the cellular and genetic process involved in tissue regeneration. Surprisingly, ascidian regeneration studies are based on only a few model species. In this chapter, we point out the important potential of solitary ascidians in regenerative and stem cell studies. We review recent studies of regeneration among solitary ascidians and discuss the cellular mechanism of tissue regeneration and the possible involvement of circulating cells in these processes. New data regarding the relationship between age and regeneration abilities of the solitary ascidian Polycarpa mytiligera (Stolidobranchia, Styelidae) are presented. The unique regeneration abilities found in P. mytiligera following evisceration of its digestive system and following amputation of its neural complex and siphon-associated structures and nerves imply on its potential to serve as a novel model system for understanding tissue regeneration.

A major focus of invasion biology is understanding the traits associated with introduction success. Most studies assess these traits in the invaded region, while only few compare nonindigenous species to the pool of potential invaders in their native region. We focused on the niche breadth hypothesis, commonly evoked but seldom tested, which states that generalist species are more likely to become introduced as they are capable of thriving under a wide set of conditions. Based on the massive introduction of tropical species into the Mediterranean via the Suez Canal (Lessepsian migration), we defined ascidians in the Red Sea as the pool of potential invaders. We constructed unique settlement plates, each representing six different niches, to assess ascidian niche breadth, and deployed them in similar habitats in the native and invaded regions. For each species found on plates, we evaluated its abundance, relative abundance across successional stages, and niche breadth, and then compared (1) species in the Red Sea known to have been introduced into the Mediterranean (Lessepsian species) and those not known from the Mediterranean (non‐Lessepsian); and (2) nonindigenous and indigenous species in the Mediterranean. Lessepsian species identified on plates in the Red Sea demonstrated wider niche breadth than non‐Lessepsian species, supporting the niche breadth hypothesis within the native region. No differences were found between Lessepsian and non‐Lessepsian species in species abundance and successional stages. In the Mediterranean, nonindigenous species numerically dominated the settlement plates. This precluded robust comparisons of niche breadth between nonindigenous and indigenous species in the invaded region. In conclusion, using Red Sea ascidians as the pool of potential invaders, we found clear evidence supporting the niche breadth hypothesis in the native region. We suggest that such patterns may often be obscured when conducting trait‐based studies in the invaded regions alone. Our findings indicate that quantifying the niche breadth of species in their native regions will improve estimates of invasiveness potential.

Coral-reef community structure has generally changed following declines in coverage by reef-building organisms. Ascidians as fast-growing, filter-feeding organisms, are of concern due to their invasiveness potential, but few studies have examined their life-history traits in tropical environments. The present study examined the distribution, settlement patterns, and reproductive cycle of the solitary, hermaphroditic ascidian Halocynthia spinosa in Eilat, Red Sea (29°30′N, 34°55′E), from August 2013 to August 2015. This species has a native distribution in the western Indian Ocean, with a continuous presence along the southern and eastern coasts of Africa. In Eilat, the field survey found that H. spinosa was prevalent in shallow depths, on cryptic, shaded substrates. Histological analysis of spermatogenesis and oogenesis, and a determination of the gonad index in monthly samples revealed a seasonal reproductive cycle that was significantly correlated with seawater temperature, associated with photoperiod and food availability. All measures of reproduction showed the highest activity in the summer (June–August), and a short resting period in the winter months (December–March). H. spinosa individuals were rarely observed on artificial structures. Thus, the introduction of H. spinosa into new regions as part of a fouling community is relatively unlikely. These findings contribute to understanding ecological interactions of non-reef-building organisms such as ascidians, and add to the knowledge of ascidian reproductive patterns.